Light valve



FIPSIOJ JUIN? 4, 1945- w. D, HERSHBERGER 2,401,425

LIGHT VALVE Filed Feb. 13. 1945 w; I l. 251' [27 M A -v INVENTOR.

BY CWM l l i i x Patented June '4, 1946 LIGHT VALVE William D. 'Hershbergen Princeton, N. J.. assigner to Radio Corporation of America, a corporation of Delaware Original application June 15, 1944, Serial No.

540,428. Divided and this application Februs PATENT OFFICE ary 13, 1945, Serial No. 577,710

' 7 Claims. (Cl- 179-1003) l This application is a division of my copending application Serial No. 540,428 filed June 15, i944, entitled Light valve, and assigned to the same assignee as the instant application.

2 heating of the gas by molecular resonance effects incidental to the excitation of the energy levels of the gas molecules. It is known that the microwave energy absorption in the gas increases as a function of the gas pressure. 'I'he variation This invention relates generally to microwave 6 transmission' and more particularly to improved in refractive index of a light valve of the micromethods of and means for controlling the denecwave absorptive gas typealso is increased as a tion of a light beam by varying the refractive function of the intensity or concentration of the index of a microwave absorptive gas interposed microwave irradiation of said gas. in the path of the light beam. i l A preferred embodiment of the invention com- The invention utilizes the characteristics of prises a cavity resonator having a microwave persome gases which are substantially perfect dimeable window opening into a waveguide which electrics at most radio frequencies but which abintroduces miclOWVe energy into the resonator. sorb considerable energy at certain other prede The resonator. if desired. may include Conductive termined microwave frequencies. For example, lo projecting members which concentrate the rein an article by cleeton and williams ln Physical soltent electric eld within a predetermined re- Review 45, 234 1934), robservations on microsion of the interior of the cavity resonator. Miwave absorption in ammonia gas indicated that oroweve absorotive gas. such, for exemple. es amradiation having a wavelength of 1.25 centimemonili. le lntIOfliolCed into the Cavity resonator Bt ters will lose approximately 63 percent of its lnizo the desired pressure to provide the required sential energy upon passing through 1.1 meters of sitivity. Light permeable windows disposed on ammonia gas in a non-metallic container at a`t` opposite 18088 0f the Cavity resonate? 'tiflaeent mospheric pressure. It was noted further that the conductive projecting elements Permit the the absorption frequency band is relatively wide focusing of a light benin thiOuBll the avlty Tessince the absorption coemcient falls to approxi- 2s Dna-tor beiieel'il the Conductive projecting elemateiy one-half of its maximum value at wavements whereby the light beam penetrates the E88 lengths of 1 centimeter and 1.5 centimeters. The 15111011811 region subjected to the maximum elecobservations described in the articie ioentlned trio field within the cavity resonator. If the conheretofore were inspired by earlier general theoductlve Projecting elements are triangular in reticalwork on the energy levels of the ammonia clOSS-Setln 0l rectangular with feces other molecule together with observations on the infra than Dlllel t0 the light permeable WindOWS. the red spectrum of this gos, but in on such prior microwave field between the conductive elements experiments no attempt was made to determine, Provides e region having a relatively abrupt explain or utilizo the effect, upon the gos of the change in refractive index which will deect or microwave absorption by said gas, retract a light beam directed therethrough.

The instant invention is related to the lnven- AIU Well known means may be employed for tion described in appuoont's oopepdmg pump introducing and venting the desired microwave tion serial No. 531,960, filed Moy 29,1944.wnere obsofptive 88S in the cavity resonator. Also. if in the change in pressure of a microwave ab desired the Cavity resonate? may be tuned t0 the sorptlve gas in response to microwave irradiation 40 Operating microwave frequency by any oonvenis utilized to provide a novel method of and means tional tuning WNW tuning P1118 01 other known for measuring microwave energy as a function of meanssimilarly ,by means 0f additional Bdthe change, or rate of change, of the pressure of lustable reactive elements Such 95 til-ning Screws said gas. 01' tuning Plugs disposed in the waveguide ad- The present invention utilizes the variation in '45 jacent the Windw loto the cavity resonator, the the refractive index of a microwave absorptive microwave resonator impedance may be matched gas in response to pressure changes therein due to the characteristic impedance of the waveguide to microwave irradiation, to provide o novelond to minimize wave reflections from the resonator. efficient light valve. In effect, the invention com- AS Wlll be explained in greater detail hereinprises a novel light prism, the refractive index of after, a microwave responsive iight valve employwhich may be directly controlled as a function of ing the novel features of the invention may be microwave irradiation, l utilized for refracting or deflecting a light beam It is believed that the variation in refractive for .indicating directly 0n en djneent indicator index of a microwave absorptive' gas subjected to scale the microwave energy absorbed inthe micropredetermined microwave irradiation is due to wave absorptive gas chamber, thereby providing an extremely convenient, accurate, and ilexillle.V

microwave wattmeter. Also. the invention may be employed for oscillagraphic or sound'iilm re .cording purposes wherein the light valve is interposed between a light source and a screen or moving photographic nlm. In this mlodincationy of the invention the deilection or refraction of the light beam may be employed in cooperation with a ixed aperture device to vary the amount of light or the position of the light beam which is tocusedupon the oscillagraphic screen or moving photographic nlm.

Among the objects of the invention are to prol vide an improved method of and means for measuring microwave energy. Another object of the invention is to provide a novel method of and means for controlling the light refractive index of a microwave absorptive gas. An additional obiect of the invention is to provide an improved method of and means for controlling the refraction or deflection of a light beam in response to the magnitude of microwave irradiation of a microwave absorptive gas.

Another object of the invention is to provide an improved method of and means for controlling a light beam. A further object of the invention is to provide an improved light valve which may be employed in cooperation with a ilxed aperture device to provide convenient and efllcient means for modulating a light beam in response to the modulation of a source of microwaves'. .A further object of the invention is to provide an improved microwave wattmeter. A still further object of 4 Y energy is absorbed within the resonator. The cavity resonator may be tuned to the desired applied frequency by means of a tuning plunger, or tuning screws, of any type well known in the art. The tuning adjustments may be made through gas-tight gaskets in the cavity resonator wall.

or by means of Sylphon ioints common to cong der substantially to prevent wave reflections and the invention is to provide an improved method of and means for recording sound on a moving motion picture film comprising a microwave responsive light valve, means for focusing a light beam through said light -valve and through a nxed aperture device to said illm, and means for varying the refractive index of said light valve in response to modulated microwaves characteristic of the sound to be recorded upon said i'ilm.

The invention will be described in greater detail by reference to the accompanying drawing of which Figure 1 is a schematic plan cross-sectional view, taken along the section line I, I of one embodiment of the invention, Figure 2 is a crosssectlonal elevational view taken along the section line 1I, 1I of said embodiment of the invention. Figure 3 is a schematic plan cross-sectional view of a second embodimnt of the invention adapted to the recording of sound on nlm, Figure 4 is a fragmentary view of a fixed aperture plate and a movable nlm forming a portion of the system of Figure 3, and Figure 5 is a schematic plan crosssectional view of a third embodiment of the invention. Similar reference characters are applied to similar elements throughout the drawing.

Either tuned or untuned cavity resonators. into which predetermined microwave energy absorbent gases may be introduced at predetermined pressures, may be employed to conne the active element to provide variations inl the refractive index of the light valve. For the purpose of illustration, it will be assumed that the cavity resonatar is filled with ammonia gas. However. various other types ol gases which absorb energy in the microwave frequency range will be listed hereinafter. A

In the tuned cavity resonator type of light valve illustrated in Figs. l, 2 and 3,' the microwave neld, to which the ammonia gas is subjected, conforms to the customary modes found in relatively sharply tuned cavity resonators, and is of high thereby to insure that most of the transmitted energy is confined to the cavity resonator and absorbed by the gas therein.

An untuned cavity resonator of the type illustrated in Fig. 3 is proportioned so that, in view of the Q of the device, which is determined by lthe resonator wall losses `and the losses in the ammonia gas, the resonant modes are so closely spaced as to overlap. This condition may be achievedby selecting the volume of the resonator to be larger than some minimum value in view of the expected Q of the resonator.

The number of resonator .modes An lying ln the frequency range Af, which is determined in turn by the value of Qin the relation -l .f Q

is approximately (l) Ari-853 c) Tp where p is thel static pressure and T is the a lute temperature.

Hence, in accordance with known relations, it1

will be understood that the variation in the refractive index of the enclosed gas will be a function of the variation in pressure of the gas in response to absorbed microwave energy. Since the refractive index of the gas also will be ccntrolled indirectly by low losses in the cavity resonator, and by the heat transferred from the gas to the cavity resonator walls, it may be found to be desirable to thermally insulate or to control, in any known manner, the temperature of the resonator walls.

The energy directly absorbed by the gas from the microwave transmission system provides a substantially rapid increase in gas temperature,

and hence, in the refractive index of the gas,

since the gas has a relatively low heat capacity and a relatively high temperature coei'iicient of expansion. These features. therefore, will prointensity since practically all of the microwave 75 vide relatively high variations in the refractive a v L l index of the enclosed gas in response to modulated microwave energy irradiatin'g 'said gal.

Unless the cavity resonator walls are thermally insulated from the enclosed gas, or are maintained at substantially constant temperature. by means of an air blast or other known expedient. the heat transfer between the cavity walls and the enclosed gas may provide relatively slow, protracted pressure variations in the aas which will be reflected as undesired variations in the refractive index thereof.

The ratio of the energy dissipated in the gas proper to the energy dissipated in the resonator walls is a function of the initial pressure of the gas confined within the resonator. Also, it depends upon proper design of the cavity itself and its input iris.

It should be understood that thecavity resonator, when properly matched to the waveguide system. performs substantially as a perfectly matched load which absorbs all of the microwave energy introduced thereto. Since, -as explained heretofore, conductive elements connected to the inner walls of the cavity resonator may be employed to concentrate the electric field within the resonator in a predetermined desired region, the overall emciency of the device may be adiusted to a relatively high value. As will be explained in greater detail hereinafter, the resultant microwave absorptive gas-tight light valve may be designed to provide single or multiple angular or lateral deflections of an applied light beam. which may be utilized in any known manner to provide desired indications of the variations in the refractive index of the gas.

Referring to Figure l of the drawing, a conventional rectangular waveguide I opens, through an aperture l, into a cavity resonator B. A gastight, microwave-permeable window l covers the aperture l. An inwardly projecting conductive element 1, having triangular cross-section, is fastened to the lower inner wide face of the cavity resonator 8. A similar inwardly-projecting, triangular cross-sectional member l. shown in Fig. 2, cooperates with the lowerl projecting element 'I to form a gap II providing a region having a concentrated electric eld in response to microwave energy introduced into the cavity resonator from the wave-guide.

Apertures Il, il, in opposite side faces of the cavity resonator 5. are covered by gas-tight, lightpermeable windows I1, I9, respectively. A light source 2i, which may include a condensing lens system 2l, directs a light beam, indicated by the dash lines' 2S or 21, through the windows in the cavity resonator 5, transversely of the gap Il between the inwardly projecting conductive elements I and l.

Microwave energy from a microwave generator (not shown) is applied to the cavity resonator l through the waveguide I and the microwavepermeable window I to establish a microwave held within the cavity resonator which is substantiaily concentrated in the gap Il between the conductive elements I and 9. The cavity resonator 8 encloses a microwave absorptive gas. such, for example, as ammonia. The gas may be maintained at any desired pressure to provide the desired sensitivity. The concentrated microwave field established between the inwardly projecting elements 'I and I provides a region of increased gas pressure in the gap II due to heating of the gas from the absorbed microwave energy incident to the concentration of the electric field in said gap. The increased gas pressure in e n l the gap II results in a corresponding variation in the iight refractive index of the ammonia gas in said gap. Hence, the boundary region between the gas in the gap Ii and the gas in the .remainder of the cavity resonator 5 e'ectiveiy -window II, may be employed to focus the light beams 2l or 21 upon a suitable calibrated scale Il, whereby the microwave power absorbed in the cavity resonator S may be indicated directly.

Referring to Fig. 2, the cavity resonator l may be tuned to resonate to the applied microwave energy by means of a conventional tuning screw 3l, which preferably includes a gas-tight rubber gasket Il. The ammonia gas may be introduced into the cavity resonator l through a conventional gas valve 31, disposed adjacent a gas intake aperture Il in one of the walls of the cavity resonator l. In 4order to match enectively the impedance of the cavity resonator I to the surge impedance of the waveguide l to prevent wave renections from the cavity resonator to the microwave energy source. tuning screws Il Il, or other conventional adjustable reactive means, may be disposed on the waveguide walls adjacent to the microwave permeable window I.

It should be understood that while the device disclosed in Figs. 1 and 2 is described as a wattmeter for the direct measurement of microwave energy, that a similar device may be employed Y to record the modulation characteristics of microwave energy upon a suitable moving screen or photographic film for osciliographlc or related purposes. i

Referring to Figs. 3 and 4, a light valve is illustrated which provides a parallel displacement of a light beam in response to variations in the magnitude of microwave energy introduced into a gas-filled cavity resonator. In this second embodiment of the invention, the structure may be identical to that described heretofore in Figs. 1 and 2, with the exception that the inwardly proiecting conducting elements disposed within the cavity resonator are substantially rectangular incross-section, and are disposed in a manner whereby their side faces form acute angles with the light permeable windows I1. Il. A light beam. derived from the light source 2| and condenser lens system 23, is introduced into the cavity resonator I through the input light window I1, to provide parallel displacement of the light beams 2B or 2l in response to different magnitudes of microwave energy introduced into the cavity resonator. f

If desired, the parallel displaced light beams 25 or 2l. which pass through the output light window Il, may be focused by means .of the projection lens 2l upon an apertured mask Iii. Light which penetrates the aperture l1 in the apertured mask 4l may, if desired, be focused by means of a supplementary `projection lens system 4l upon the desired area of a moving photographic tllm Il.

The device thus described may be employed for the purpose of recording signal modulation upon a moving photographic film such. for example, as in motion picture sound recording. For this purpose, the microwave energy introduced into the waveguide I is amplitude-modulated in any known manner by means of the desired modulation signals to provide a light beam of variable area, which may be recorded upon the moving nlm II. The dashed circles 53, 55, adjacent the aperture W in the apertured plate 45, indicate the manner in which the light beam derived from the cavity resonator light valve is masked by the. masking plate 45. It should be understood that, alternatively, the device described in Fig. 3 may be used for the direct measurement of microwave power by focusing the output light beam upon a suitably calibrated scale, as described heretofore in Figs. l and 2.

Referring to Figure 5. a. third embodiment of the invention is illustrated wherein the inwardlyprojecting conductive elements described heretofore are omitted, and wherein the light permeable windows Il. I9 are disposed at current nodes in opposite walls of the cavity resonator 5. In this third embodiment of the invention the cavity resonator comprises a section of the waveguide I, having an effective length of three half-wavelengths. 'I'he tuning screw 33 may be located on one of the side walls of the cavity resonator 5 or, if desired, may be disposed in the end wall 51 closing the end of the waveguide I at a point three half-wavelengths from the input aperture 3 and microwave permeable window 8..

The light permeable windows I1, I9 are disposed in the narrow side walls of the cavity resonator 5. at points separated one-half wavelength from either end of the cavity resonator, and thus, one-half wavelength from each other.

'l'i'ius, a light beam, derived from the light source 2| and condensing lens system 23, is di-` rected diagonally through the gas prism formed by the gas-nlled cavity resonator 5, and may be focused by the projection lens 29 upon a calibated scale li, for the direct measurement of the magnitude of the microwave energy introduced into th'e cavity resonator.

Alternately, the latter device may be employed, as described heretofore in Figs. 3 and 4, for the recording of light variations upon a moving photographic nlm. v

It should be understood that the sensitivity of the embodiment of the invention illustrated in Fig. 5 will be less than that of the embodiments described in Figs. l, 2 and 3, since the electric field within the cavity resonator will not be concentrated in the region through which the light beam is directed. However, the embodiment shown in Fig. 5 provides a convenient construction, especially in instances wherein conventional 1.25 centimeter waveguides are employed, since the physical size of such waveguides seriously limits the use of internal structural elements. Furthermore, the embodiment of the invention illustrated in Fig. 5 has the advantage that the light permeable windows II, I9 are disposed at points of low magnetic field intensity, thereby minimizing microwave energy leakage from th cavity resonator I.

It should be widerstand that the impedance of the cavity resonator I may be matched to the surge impedance of the waveguide I in the same manner as described heretofore in Figs. l, 2 and 3, in order to minimize wave reflections from the cavity resonator to the microwave generator.

Various other microwave absorptive gases have been tested and found to be quite satisfactory for microwave power measurements in apparatus of have been measured:

Thus the invention described comprises several embodiments of an improved light valve which may be employed for the measurement or indi.- cation of microwave energy or signal modulation of said energy. Th'e devices disclosed provide an extremely convenient, accurate, and vsensitive means for measuring or indicating microwave energy in the millimeter and centimeter regions, wherein the invention eilectively comprises a gas type prism, the refractive index of which may be varied as a function of the magnitude applied microwave energy.

I claim as my invention:

l. A microwave responsive recording system including. a microwave resonant chamber for enclosing a microwave energy absorptive gas, means for introducing microwave energy modulated by signal intelligence into said chamber to establish a microwave field in said gas for varying the light refractive properties of said gas in response to said signal intelligence, a light beam source, means for directing said light beam throughl said variable refractive gas in a manner whereby said beam is refracted as a function of the intensity of said field, a xed light hunting device, a movable photo-sensitized element, and means for directing said variabb' refracted light through said light limiting device to said movable element to provide thereon'a photographic image characteristic of said signal intelligence.

2. A microwave responsive recording system including a microwave resonant chamber for enclosing a microwave energy absorptive gas, a microwave permeable window in said chamber, means for introducing microwave energy modulated by signal intelligence through said window into said chamber to establish a microwave field in said gas for varying the light refractive properties of said gas in response to the energy absorbed by said gas. a light beam source, means for directing said lightv beam through said variable refractive gas in a manner whereby said beam is refracted as a function of the intensity of said eld, a fixed light limiting device, amovable photo-sensitized element, and means for directing said variably retracted light through said light limiting device to said movable element to provide thereon a photographic image characteristic of said signal intelligence.

3. A microwave responsive recording system including a microwave resonant chamber for enclosing a microwave energy yabsorptive gas, a microwave permeable window in said chamber, means for introducing microwave energy modulated by signal intelligence through said window into said chamber to establish a lmicrowave ileld in said gas for varying the light refractive prop- 76 erties of said gas in response to the energy abable photo-sensitized element, and means for directing said variably deiiected light through said light limiting device to said movable element t0 provide thereon a photographic image characteristie of said signal intelligence.

4. A system as described in claim 3 including a lens disposed intermediate said chamber and said photo-sensitized element for imagin said detlected iight beam on said element.

5. A microwave responsive recording system includling a microwave resonant chamber for enclosing a microwave energy absorptive ga's, a microwave permeable window in said chamber. a pair of conductive elements extending toward each other from opposite sides of said chamber and forming a gap between adjacent ends thencot. means tor introducing microwave energy modulated by signal intelligence through said window into said chamber to establish a microwave field in said gas in said gap for varying the light re- -tractive properties ot said gas in said gap in response to the energy absorbed by said gas, a light beam source. means for directing said light beam through said variable refractive gas in said gap in a manner whereby said' beam is retracted as a function of the intensity of said ileid, a nxed light limiting device. a movable photo-sensitized element, and means tor directing said variably retracted light through said light limiting device to said movable element to provide thereon a photographic image characteristic of said signal intelligence. I

6. A microwave responsive recording system including a microwave resonant chamber for enclosing a microwave energy absorptive gas. a microwave permeable window in said chamber. means for introducing microwave energy mod- 0 ulated by signal intelligence through said window into said chamber to establish a microwave eld in said gas for varying the light refractive propertieg ot said gas in response to the energy absorbed by said gas. a light beam source. means for directing said light beam through said variable refractive gas in a manner whereby said beam is retracted on an axis parallel to its original axis as a function ot the intensity of said 1o field, a fixed light limiting device, a movable photo-sensitized element, and means for directing said variably retracted light through said light limiting device to said movable element to provide thereon a photographic image characteristic of said signal intelligence 7. A microwave responsive sound recording system including a microwave resonant chamber for enclosing a microwave energy absorptive gas, a microwave permeable window in said chamler. a

2() pair of rectangular conductive elements extending toward each other from opposite sides of said chamber and having parallel disposed faces forming a gap therebetween, means .for introducing microwave energy modulated by sound signal intelligence through said window into said chamber to establish a microwave ileld in said gas in said gap for varying the light refractive properties ot said gas in said gap in response .to ,the energy absorbed by said gas, a light beam ao'source. means for Adirecting said light beam through said variable refractive gas in said gap in a manner whereby said beam is retracted on an axis parallel to its original axis as a function of the intensity of said field, a ilxed light limiting device, a movable photo-sensitized element,

and means tor directing said variably retracted light through said iight limiting device to said movable element to provide thereon a photographic image characteristic ot said sound sig- 40 nalintellim.

WILLIAM D. HERSHBERER. 

